Silicon composite, making method, and non-aqueous electrolyte secondary cell negative electrode material

ABSTRACT

A silicon composite comprises silicon particles whose surface is at least partially coated with a silicon carbide layer. It is prepared by subjecting a silicon powder to thermal CVD with an organic hydrocarbon gas and/or vapor at 900-1,400° C., and heating the powder for removing an excess free carbon layer from the surface through oxidative decomposition.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. §119(a)on patent application No. 2004-195586 filed in Japan on Jul. 1, 2004,the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to a silicon composite powder having a capacitycontrolled to compensate for the drawback of silicon which is believeduseful as lithium ion secondary cell negative electrode active material;a method for preparing the same; and a non-aqueous electrolyte secondarycell negative electrode material comprising the powder.

BACKGROUND ART

With the recent remarkable development of potable electronic equipment,communications equipment and the like, a strong demand for high energydensity secondary batteries exists from the standpoints of economy andsize and weight reductions. One prior art method for increasing thecapacity of secondary batteries is to use oxides as the negativeelectrode material, for example, oxides of V, Si, B, Zr, Sn or the likeor complex oxides thereof (see JP-A 5-174818 and JP-A 6-060867corresponding to U.S. Pat. No. 5,478,671), metal oxides quenched fromthe melt (JP-A 10-294112), silicon oxide (Japanese Patent No. 2,997,741corresponding to U.S. Pat. No. 5,395,711), and Si₂N₂O and Ge₂N₂O (JP-A11-102705 corresponding to U.S. Pat. No. 6,066,414). Conventionalmethods of imparting conductivity to the negative electrode materialinclude mechanical alloying of SiO with graphite, followed bycarbonization (see JP-A 2000-243396 corresponding to U.S. Pat. No.6,638,662), coating of silicon particles with a carbon layer by chemicalvapor deposition (JP-A 2000-215887 corresponding to U.S. Pat. No.6,383,686), and coating of silicon oxide particles with a carbon layerby chemical vapor deposition (JP-A 2002-42806). None of these patentsrelates to the method of alleviating a substantial volume change of asilicon negative electrode during charge/discharge cycles which is anoutstanding problem characteristic of the silicon negative electrode northe method of reducing the current collection associated with the volumechange. It remains an important task to establish such techniques.

The prior art methods using silicon as such or making its surfaceconductive for improving the cycle performance of negative electrodematerial are successful in increasing the charge/discharge capacity andenergy density, but are not necessarily satisfactory because of failureto fully meet the characteristics required in the market, particularlythe cycle performance of importance in mobile phone and otherapplications. There is a desire for further improvement in cycleperformance.

In particular, Japanese Patent No. 2,997,741 uses silicon oxide as thenegative electrode material in a lithium ion secondary cell to providean electrode with a high capacity. As long as the present inventors haveconfirmed, there is left a room for further improvement as demonstratedby a still high irreversible capacity on the first charge/dischargecycle and cycle performance below the practical level. With respect tothe technique of imparting conductivity to the negative electrodematerial, JP-A 2000-243396 suffers from the problem that solid-to-solidfusion fails to form a uniform carbon coating, resulting in insufficientconductivity. In the method of JP-A 2000-215887 which can form a uniformcarbon coating, the negative electrode material based on siliconundergoes excessive expansion and contraction upon adsorption anddesorption of lithium ions, meaning impractical operation, and losescycle performance. Thus, the charge/discharge quantity must be limited.In JP-A 2002-42806, despite a discernible improvement of cycleperformance, due to precipitation of silicon crystallites, insufficientstructure of the carbon coating and insufficient fusion of the carboncoating to the substrate, the capacity gradually lowers ascharge/discharge cycles are repeated, and suddenly drops after a certainnumber of charge/discharge cycles. This approach is thus insufficientfor use in secondary cells.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a silicon compositewhich maintains the high initial efficiency inherent to silicon, hasexcellent cycle performance, and has alleviated a substantial volumechange during charge/discharge cycles so that it is effective as activematerial for lithium ion secondary cell negative electrodes; a methodfor preparing the same; and a non-aqueous electrolyte secondary cellnegative electrode material comprising the silicon composite.

The inventor has discovered a silicon composite which maintains the highinitial efficiency inherent to silicon, has excellent cycle performance,and has alleviated a substantial volume change during charge/dischargecycles and which is thus effective as the active material for lithiumion secondary cell negative electrodes.

The development of an electrode material having an increasedcharge/discharge capacity is very important and many engineers have beenengaged in the research and development thereof. Under thecircumstances, silicon and amorphous silicon oxides represented by thegeneral formula SiO_(x) wherein 1.0≦x<1.6 are of great interest as thenegative electrode active material for lithium ion secondary cellsbecause their capacity is large. Only few of them have been used inpractice because of their shortcomings including substantial degradationupon repeated charge/discharge cycles, that is, poor cycle performanceand in particular, low initial efficiency.

Making investigations from such a standpoint with the target ofimproving cycle performance and initial efficiency, the inventor foundthat CVD treatment of silicon oxide powder led to a substantialimprovement in performance as compared with the prior art. However, thisapproach starting with silicon oxide left the problem of low initialefficiency due to the presence of oxygen atoms. The problem could, ofcourse, be solved by some means, for example, by the addition of phenyllithium which is known as a method for compensating for the low initialefficiency. These solutions, however, invited side issues that the cellmanufacture process becomes complex and unnecessary materials are leftwithin the cell.

In contrast, a silicon powder characterized by the absence of oxygen isexpected to have a far greater charge/discharge capacity than thesilicon oxide. On the other hand, the silicon powder undergoes asubstantial volume change while occluding and releasing a large amountof lithium, which can cause separation between silicon and binder,breakage of silicon particles, and even separation between the electrodefilm and the current collector. This leads to such problems as a failureof current collection and cycle degradation. Among approachescontemplated to solve these problems, electrically controlling thecapacity is effective as a method of alleviating the volume change byexpansion and contraction of silicon, but impractical. Under thecircumstances where a charge/discharge capacity as large as that ofsilicon is not necessary, there is a need for a silicon base materialhaving the advantages of a less volume change and good adhesion tobinders or the like, notwithstanding a lower energy density thansilicon.

Making extensive investigations from this standpoint on a material whichundergoes a less volume change upon occlusion and release of lithiumeven during full charge/discharge operation and has highly adhesivesurfaces, the inventor has found that the above problems of lithium ionsecondary cell negative electrode active material are overcome bycoating silicon particles or micro-particles with an inert robustmaterial, that is, silicon carbide. The resulting material has aninitial efficiency comparable to or surpassing the existing carbonaceousmaterials and an extremely greater charge/discharge capacity than thecarbonaceous materials and achieves drastic improvements in cycliccharge/discharge operation and efficiency thereof.

In one aspect, the present invention provides a silicon compositecomprising silicon particles whose surface is at least partially coatedwith a silicon carbide layer.

In preferred embodiments, the silicon particles have an average particlesize of 50 nm to 50 μm; the silicon particle surface is at leastpartially fused to silicon carbide; a diffraction line attributable tosilicon is observed when the silicon composite is analyzed by x-raydiffractometry; the silicon composite contains free carbon in an amountof up to 5% by weight; the silicon composite contains 5 to 90% by weightof zero-valent silicon capable of generating hydrogen gas when reactedwith an alkali hydroxide solution; and after the silicon composite istreated with a mixture of hydrofluoric acid and an oxidizing agent andheat dried, silicon carbide is left as the evaporation residue.

In another aspect, the present invention provides a method for preparingthe silicon composite defined above, comprising the steps of subjectinga silicon powder to thermochemical vapor deposition treatment with anorganic hydrocarbon gas and/or vapor at 900° C. to 1,400° C., andheating the powder for removing a surface excess free carbon layerthrough oxidation.

Also contemplated herein is a negative electrode material for anon-aqueous electrolyte secondary cell, comprising the silicon compositedefined above.

The silicon composite of the present invention maintains the highinitial efficiency inherent to silicon, has excellent cycle performance,and alleviates a substantial volume change during charge/dischargecycles so that it is effective as the active material for lithium ionsecondary cell negative electrodes. When the silicon composite is usedas the active material for a lithium ion secondary cell negativeelectrode, the resulting lithium ion secondary cell negative electrodematerial is adherent to a binder, has a high initial efficiency,alleviates a volume change during charge/discharge cycles, and isimproved in repeated cyclic operation and efficiency thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 diagrammatically illustrates in cross section the structure of asilicon composite particle of the invention.

FIG. 2 is a chart of x-ray diffraction (Cu—Kα) on a silicon compositeobtained by starting with a silicon powder, conducting thermal CVD usingmethane gas, and conducting oxidative decomposition for removing carbon.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For use as the lithium ion secondary cell negative electrode activematerial a siliceous material is expected promising because of itscharge/discharge capacity which is several times greater than that ofthe current mainstream graphite-derived materials, but is prevented frompractical use by the degradation of performance with repeatedcharge/discharge operation and a substantial volume change duringcharge/discharge operation. The present invention relates to a siliconcomposite which maintains the high initial efficiency, improves thecycle performance, and alleviates the volume change of the siliceousmaterial. The silicon composite is arrived at by effecting chemicalvapor deposition on silicon micro-crystals for coating their surfaceswith carbon and forming a silicon carbide layer at the interface, andthen removing the carbon layer on the surfaces through oxidation.

More particularly, the silicon powder used herein is metallic silicon ofthe industrial grade, polycrystalline silicon for semiconductor orsemiconductor single-crystal powder which has been previously pulverizedto a predetermined particle size, or a microparticulate silicon powderwhich has been prepared by inert gas quenching, gas phase precipitationor the like. A hydrocarbon compound gas and/or vapor is introduced,after which thermochemical vapor deposition treatment is effected on thesilicon powder at 900° C. to 1,400° C., thereby forming a carbon layeron the silicon surface and at the same time forming a silicon carbidelayer at the interface between the silicon and the carbon layer.Subsequent heating in an oxidizing atmosphere causes oxidativedecomposition of the carbon layer whereby the carbon layer is removed,leaving the silicon particles whose surface is at least partially coatedwith silicon carbide. In this way, silicon carbide-surface-coatedsilicon particles, referred to herein as silicon composite particles,are readily prepared. In the silicon composite particles, it ispreferred that at least part of the silicon surface be fused to siliconcarbide. The outermost coating may be of an electrically conductivematerial which is not decomposed or altered in the cell although siliconcarbide is most preferred from the adhesion standpoint as well.

The silicon composite of the invention should preferably meet thefollowing conditions.

i) By varying the temperature and time of chemical vapor depositiontreatment, the thickness and proportion of the silicon carbide layer canbe controlled. With this technique, therefore, the quantity ofzero-valent silicon serving as a lithium occluding mass, that representsa charge/discharge capacity, can be controlled, and the volume changeassociated with charge/discharge cycles is eventually alleviated.

ii) In x-ray diffractometry (Cu—Kα) using copper as a counter cathode, adiffraction pattern attributable to silicon, for example, a diffractionpeak centering approximately at 2θ=28.4° and attributable to Si(111) isobservable.

iii) The quantity of zero-valent silicon capable of occluding andreleasing lithium ions when used in a lithium ion secondary cellnegative electrode can be measured in terms of the quantity of hydrogengenerated upon reaction with an alkali hydroxide according to ISO DIS9286, the method of measuring free silicon in silicon carbide finepowder. As calculated from the quantity of hydrogen generated,zero-valent silicon is contained in an amount of 5 to 90% by weight,more preferably 20 to 80% by weight, and even more preferably 30 to 60%by weight of the silicon composite.

iv) When the silicon composite is dissolved in hydrofluoric acidcontaining an oxidizing agent such as hydrogen peroxide, silicon carbidesettles down without being dissolved. When this solution is evaporatedto dryness, a green tinted residue is left. The silicon composite hasthe structure shown by a schematic image of FIG. 1.

v) When used as a lithium ion secondary cell negative electrode activematerial, the silicon composite has an initial efficiency equal to orgreater than that of the current graphite-derived materials andundergoes a reduced volume change and minimized cycle degradation ascompared with silicon itself.

In the silicon composite powder of the invention, the quantity of freecarbon and the quantity of carbon in silicon carbide form are determinedfrom the quantity of free carbon and a difference between the totalcarbon quantity and the free carbon quantity as measured according toJIS R-6124. The quantity of silicon carbide coated is determined bymultiplying the quantity of carbon in silicon carbide form by40/12=3.33.

Preference is given to a less quantity of free carbon relative to thesilicon composite powder (i.e., silicon composite powder surface coatedwith silicon carbide, prepared by thermal CVD and subsequent oxidativedecomposition). Specifically, the quantity of free carbon is preferablyup to 5% by weight (5 to 0% by weight), more preferably up to 3% byweight (3 to 0% by weight) and even more preferably up to 2% by weight(2 to 0% by weight) of the silicon composite. The quantity of siliconcarbide coated is preferably 10 to. 95% by weight, more preferably 20 to80% by weight, and even more preferably 40 to 70% by weight of thesilicon composite. If the quantity of silicon carbide coated is lessthan 10 wt % of the silicon composite, the silicon composite may haveinsufficient cycle performance when incorporated in a lithium ionsecondary cell. If the lo quantity of silicon carbide coated is morethan 95 wt % of the silicon composite, the inactive silicon carbidecoating has an increased thickness, which may inhibit migration oflithium ions and reduce the negative electrode capacity.

Next, the method for preparing the electrically conductive siliconcomposite according to the invention is described.

The method starts with a silicon powder preferably having an averageparticle size of 50 nm to 50 μm which is obtained by mechanicallypulverizing metallic silicon of the industrial grade, polycrystallinesilicon for semiconductor, or semiconductor silicon, or amicroparticulate silicon powder preferably having an average particlesize of 50 nm to 10 μm, more preferably 100 nm to 5 μm, which isobtained by quenching silicon vapor with an inert gas such as argon. Ina fluidizing gas atmosphere containing at least an organic matter gasand/or vapor, thermal chemical vapor deposition (CVD) treatment iseffected on the silicon powder at a temperature of preferably 900° C. to1,400° C., more preferably 900° C. to 1,300° C. The resultingcarbon/silicon carbide coated silicon powder is heat treated in anoxidizing atmosphere, typically air, at a temperature of preferably 600to 1,400° C., more preferably 600 to 900° C., and even more preferably650 to 800° C., whereby the surface layer of free carbon is oxidativelydecomposed away. There are obtained silicon composite particles in whichat least part of SiC is fused to or merged with Si.

The silicon particles whose surface will be at least partially coatedwith silicon carbide should preferably have an average particle size of50 nm to 50 μm, more preferably 100 nm to 20 μm, even more preferably200 nm to 10 μm, and most preferably 500 nm to 5 μm. Silicon particleswith an average particle size of less than 50 nm may be difficult tohandle whereas silicon particles with an average particle size of morethan 50 μm may penetrate through the negative electrode film.

At a CVD temperature below 900° C., the formation of a silicon carbidecoating proceeds at a slow rate, takes a long time, and becomesinefficient. Inversely, at a CVD temperature above 1,400° C., silicon ismelted, deviating from a uniform particle form. With respect to theoxidative decomposition of the free carbon layer, a temperature below600° C. may induce insufficient oxidative decomposition. The temperatureand time for CVD are determined as appropriate relative to the thicknessof the carbon layer. During the CVD treatment, particles can agglomeratetogether. In such a case, the agglomerates are preferably disintegratedon a ball mill or the like in order to facilitate the subsequentoxidative decomposition. In some cases, the CVD treatment may berepeated similarly.

The organic materials used as the raw material for generating theorganic matter gas and/or vapor include those which are pyrolyzed at theheat treatment temperature, particularly in a non-oxidizing atmosphere,to form carbon or graphite, for example, hydrocarbons such as methane,ethane, ethylene, acetylene, propane, butane, butene, pentane, isobutaneand hexane, alone or in admixture, and mono- to tri-cyclic aromatichydrocarbons such as benzene, toluene, xylene, styrene, ethylbenzene,diphenylmethane, naphthalene, phenol, cresol, nitrobenzene,chlorobenzene, indene, coumarone, pyridine, anthracene, andphenanthrene, alone or in admixture. Also useful are gas light oil,creosote oil, anthracene oil and naphtha-cracked tar oil resulting fromtar distillation process, alone or in admixture.

The heat treatment of silicon powder and the organic matter gas and/orvapor is not particularly limited as long as a reactor system includinga heating mechanism is employed in a non-oxidizing atmosphere. Thetreatment may be either continuous or batchwise. Depending on aparticular purpose, a choice may be made among a fluidized bed reactor,rotary furnace, vertical moving bed reactor, tunnel furnace, batchfurnace, and rotary kiln.

The fluidizing gas used herein may be the above-described organic mattergas alone or a mixture of the organic matter gas and a non-oxidizing gassuch as Ar, He, H₂ and N₂. Preferably the linear velocity u (m/sec) ofthe fluidizing gas is controlled such that the ratio of the linearvelocity u to the minimum fluidization velocity u_(mf) may be in therange of 1.5 to 5 (i.e., 1.5≦u/u_(mf)≦5), whereby a conductive coatingis formed more efficiently. A u/u_(mf) ratio below 1.5 may lead toinsufficient fluidization, introducing variations in the conductivecoating. Inversely, a u/u_(mf) ratio in excess of 5 may allow forsecondary agglomeration of particles, failing to form a uniformconductive coating.

It is noted that the minimum fluidization velocity varies with the sizeof particles, treating temperature, treating atmosphere and the like.The minimum fluidization velocity is defined as the linear velocity ofthe fluidizing gas at which a pressure loss across the powder becomesequal to W/A wherein W is the weight of the powder and A is thecross-sectional area of the fluidized layer, when the linear velocity ofthe fluidizing gas is slowly increased the minimum fluidization velocityu_(mf) is typically in the range of about 0.1 to 30 cm/sec, preferablyabout 0.5 to 10 cm/sec. The average particle size of silicon particlesimparting u_(mf) in this range is typically 0.5 to 50 μm, and preferably1 to 30 μm. With an average particle size of less than 0.5 μm, secondaryagglomeration may take place, inhibiting effective treatment of surfacesof individual particles. Particles with an average particle size of morethan 50 μm may be difficult to uniformly coat on the surface of acurrent collector in a lithium ion secondary cell.

The thus obtained silicon composite powder of the invention has anaverage particle size of typically 0.08 to 52 μm, preferably 0.1 to 50μm, more preferably 0.5 to 40 μm, and most preferably 1 to 20 μm. Toosmall an average particle size corresponds to too large a surface area,which may lead to too low a negative electrode film density. Too largean average particle size has the risk of penetrating through thenegative electrode film. It is noted that throughout the specification,the average particle size is determined as a weight average diameter D₅₀(particle diameter at 50% by weight cumulative, or median diameter) uponmeasurement of particle size distribution by laser light diffractometry.

The silicon composite powder obtained by the invention may be used as anactive material for non-aqueous electrolyte secondary cell negativeelectrodes. Due to many advantages including a high capacity as comparedwith the existing graphite and the like, a high initial efficiency ascompared with silicon oxide or silicon oxide-derived materials, acontrolled volume change upon charge/discharge cycles as compared withsilicon, and a good adhesion between particles and a binder, the siliconcomposite powder may be used to construct a non-aqueous electrolytesecondary cell, especially lithium ion secondary cell, having improvedcycle performance.

When a negative electrode is prepared using the silicon compositepowder, a conductive agent such as graphite may be added to the siliconcomposite powder. The type of conductive agent is not particularlylimited as long as it is an electron conductive material which does notundergo decomposition or alteration in the cell associated therewith.Illustrative conductive agents include metals in powder or fiber formsuch as Al, Ti, Fe, Ni, Cu, Zn, Ag, Sn and Si, natural graphite,synthetic graphite, various coke powders, meso-phase carbon, vapor phasegrown carbon fibers, pitch base carbon fibers, PAN base carbon fibers,and graphite obtained by firing various resins.

In the embodiment wherein the conductive agent is added, the amount ofthe conductive agent is preferably 1 to 60% by weight, more preferably10 to 50% by weight, and most preferably 20 to 50% by weight of amixture of the silicon composite powder and the conductive agent. Amixture with less than 1% of the conductive agent may fail to withstandexpansion and contraction on charge/discharge cycles, whereas a mixturewith more than 60% of the conductive agent may have a reducedcharge/discharge capacity. The total amount of carbon in said mixture ispreferably 25 to 90% by weight, more preferably 30 to 50% by weight. Amixture with less than 25% by weight of carbon may fail to withstandexpansion and contraction on charge/discharge cycles, whereas a mixturewith more than 90% of carbon may have a reduced charge/dischargecapacity.

When the negative electrode is prepared using the silicon compositepowder, an organic polymer binder may be added to the silicon compositepowder. Examples of organic polymer binder include polymers such aspolyethylene, polypropylene, polyethylene terephthalate, aromaticpolyamides, aromatic polyimides, cellulose, poly(vinylidene fluoride),polytetrafluoroethylene, and copolymerized fluoro-polymers includingtetrafluoroethylene; rubbery polymers such as styrene-butadiene rubber,isoprene rubber, butadiene rubber and ethylene-propylene rubber;flexible polymers such as ethylene-vinyl acetate copolymers andpropylene-alpha-olefin copolymers; and ion conductive polymers such asorganic polymers (e.g., polyethylene oxide, polypropylene oxide,polyepichlorohydrin, polyphosphazene, polyvinylidene fluoride,polyacrylonitrile), combined with lithium salts or alkali metal saltsprimarily containing lithium.

With respect to the mixing proportion of the organic polymer binder andthe silicon composite powder, it is preferred to use 0.1 to 30 parts byweight, more preferably 0.5 to 20 parts by weight, and even morepreferably 1 to 15 parts by weight of the organic polymer binder per 100parts by weight of the silicon composite powder. Outside the range, toosmall an amount of the binder may allow silicon composite particles toseparate off whereas too large an amount may lead to a reduced percentvoid and/or a thicker insulating film, prohibiting lithium ions frommigration.

In addition to the organic polymer binder, a viscosity adjusting agentmay be added, for example, carboxymethyl cellulose, sodium polyacrylateand other acrylic polymers.

The lithium ion secondary cell-forming negative electrode material (ornon-aqueous electrolyte secondary cell-forming negative electrodematerial) of the invention can be shaped into a lithium ion secondarycell-forming negative electrode by the following exemplary procedure.For example the silicon composite powder, conductive agent, organicpolymer binder and other additives are admixed with a solvent suitablefor dissolving or dispersing the binder, such as N-methylpyrrolidone orwater to form a paste mix, which is applied in a sheet form to a currentcollector. The current collector may be copper foil, nickel foil or anyother materials which are typically used as the negative electrodecurrent collector. The method of shaping the mix into a sheet is notparticularly limited and any of well-known methods may be used.

Using the lithium ion secondary cell-forming negative electrode thusobtained, a lithium ion secondary cell can be fabricated. The lithiumion secondary cell thus constructed is characterized by the use of thesilicon composite as the negative electrode active material while thematerials of the positive electrode, electrolyte, and separator and thecell design are not critical. For example, the positive electrode activematerial used herein may be selected from transition metal oxides suchas LiCoO₂, LiNiO₂, LiMn₂O₄, V₂O₆, MnO₂, TiS₂ and MoS₂ and chalcogencompounds. The electrolytes used herein may be lithium salts such aslithium perchlorate in non-aqueous solution form. Examples of thenon-aqueous solvent include propylene carbonate, ethylene carbonate,dimethoxyethane, γ-butyrolactone and 2-methyltetrahydrofuran, alone orin admixture. Use may also be made of other various non-aqueouselectrolytes and solid electrolytes.

EXAMPLE

Examples of the invention are given below by way of illustration and notby way of limitation. In Examples, all parts and percents are by weightunless otherwise stated. The average particle size is determined as acumulative weight average diameter D₅₀ (or median diameter) uponmeasurement of particle size distribution by laser light diffractometry.

Example 1

To illustrate the structure of the silicon composite of the invention, asilicon composite was prepared using a milled powder of industrialmetallic silicon.

A metallic silicon mass of industrial grade was crushed on a crusher,and milled on a ball mill and then a bead mill using hexane as adispersing medium until fine particles having a predetermined averageparticle size (about 4 μm) were obtained. A vertical reactor was chargedwith the silicon fine powder, and thermal CVD was conducted in a streamof a methane-argon mixture at 1,150° C. for an average residence time ofabout 4 hours. The black mass thus obtained was heated in air at 800° C.for one hour for removing free carbon from the surface. After theoxidizing treatment, the mass was disintegrated on an automated mortarinto a fine powder having an average particle size of about 4 μm.

The silicon composite powder was analyzed by x-ray diffractometry(Cu—Kα). FIG. 2 illustrates an x-ray diffraction pattern, which provesthe presence of a diffraction line near 2θ=28.40 attributable to Si(111)of crystalline silicon (diamond structure). It is thus seen that finesilicon crystals were left. Also the quantity of active silicon orzero-valent silicon was measured in terms of the quantity of hydrogengenerated upon reaction with a sodium hydroxide solution according toISO DIS 9286, the method of measuring free silicon in silicon carbidefine powder. The quantity of active silicon in the silicon compositepowder was found to be about 62%, confirming the presence of zero-valentsilicon having a lithium occluding ability.

Further, an aliquot (about 2 g) of the silicon composite powder wasplaced on a platinum dish, to which a hydrofluoric acid/hydrogenperoxide mixture was added for dissolving the powder. The presence of aprecipitate was observed. The liquid mixture was evaporated to drynessby heating in a draft, with a greenish gray residue characteristic ofsilicon carbide being observed.

The quantities of free carbon and total carbon were measured to be 1.5%and 12.3%, respectively, according to JIS R-6124, the method of chemicalanalysis on silicon carbide abrasives. The difference (10.8%) betweenthem is attributed to carbon in silicon carbide form. It is thus seenthat 10.8×3.33=36.0% is present as silicon carbide.

The silicon composite powder has a structure in which the surface of asilicon particle is coated with silicon carbide as shown in FIG. 1

Example 2

A metallic silicon mass of industrial grade (low Al silicon availablefrom SIMCOA Operations Pty. Ltd., Australia, Al 0.4%, Fe 0.21%, etc.)was crushed on a jaw crusher and atomized on a jet mill, obtaining finesilicon particles having an average particle size of about 4 μm. Thesilicon fine powder was fed to a rotary kiln reactor, and thermal CVDwas conducted in a stream of a methane-argon mixture at 1,200° C. for anaverage residence time of about 2 hours. The black mass thus obtainedwas disintegrated on an automated mortar. The powder was fed to therotary kiln reactor again, and thermal CVD was conducted in a stream ofa methane-argon mixture at 1,200° C. for an average residence time ofabout 2 hours. The black mass thus obtained was similarly disintegratedon the automated mortar again. There was obtained a black powder havinga total carbon content of 47% and a free carbon content of 36%. Theblack powder was then placed in an alumina bowl, and heated in air at800° C. for one hour for oxidizing and removing free carbon on thesurface, yielding a blackish gray fine powder.

The fine powder had a total carbon content of 12.1%, a free carboncontent of 1.3%, an oxygen content of 1.5%, and an average particle sizeof 4.0 μm. On x-ray analysis, a diffraction line attributable to siliconwas found. Upon treatment with a hydrofluoric acid/hydrogen peroxidemixture, a precipitate in the solution was observed, and afterevaporation to dryness, a grayish green residue was observed. These dataconfirmed the structure in which the surface of a silicon particle iscoated with silicon carbide.

From the analytical data, the powder was found to have a composition of62.7% silicon and 36.0% silicon carbide.

Cell Test

The evaluation of silicon composite powder as the negative electrodeactive material for a lithium ion secondary cell was carried out by thefollowing procedure which was common to all Examples and ComparativeExamples. A mixture was first obtained by adding synthetic graphite(average particle diameter D₅₀=5 μm) to the silicon composite in suchamounts that the total of carbon in synthetic graphite and carbondeposited on the silicon composite was 40%. To the mixture were added3.0 pbw of carboxymethyl cellulose (ammonium salt) as a water-solublethickener, 6.0 pbw of a styrene-butadiene copolymer latex (solids 50%)as a binder using water as a dispersing medium, and 50 pbw of water. Themixture was agitated to form a slurry. The slurry was coated onto acopper foil of 20 μm gage and dried at 120° C. for one hour. Using aroller press, the coated foil was shaped under pressure into anelectrode sheet, of which 2 cm² discs were punched out as the negativeelectrode.

To evaluate the charge/discharge performance of the negative electrode,a test lithium ion secondary cell was constructed using a lithium foilas the counter electrode. The electrolyte solution used was anon-aqueous electrolyte solution of lithium phosphorus hexafluoride in a1/1 (by volume) mixture of ethylene carbonate and 1,2-dimethoxyethane ina concentration of 1 mol/liter. The separator used was a microporouspolyethylene film of 30 μm thick.

The lithium ion secondary cell thus constructed was allowed to standovernight at room temperature. Using a secondary cell charge/dischargetester (Nagano K.K.), a charge/discharge test was carried out on thecell. Charging was conducted with a constant current flow of 3 mA untilthe voltage of the test cell reached 0 V, and after reaching 0 V,continued with a reduced current flow so that the cell voltage was keptat 0 V, and terminated when the current flow decreased below 100 μA.Discharging was conducted with a constant current flow of 3 mA andterminated when the cell voltage rose above 2.0 V, from which adischarge capacity was determined.

The initial charging capacity and efficiency of this lithium ionsecondary cell were determined. By repeating the above operations, thecharge/discharge test on the lithium ion secondary cell was carried out50 cycles. The discharging capacity on the 50th cycle was measured, fromwhich a percent cycle retention (after 50 cycles) was computed. The testresults are shown in Table 1.

Example 3

The metallic silicon powder with an average particle size of about 4 μmobtained through the pulverization in Example 2 was milled on a beadmill using hexane as a dispersing medium to a predetermined particlesize (the target average particle size below 1 μm). A milled siliconpowder having an average particle size of about 0.8 μm was obtained. Theresulting slurry was passed through a filter for removing hexane,leaving a cake-like mass containing hexane, which was placed in analumina bowl. The bowl was set in a lateral reactor, and thermal CVD wasconducted in a stream of a methane-argon mixture at 1,150° C. for about2 hours. The black mass thus obtained was disintegrated on an automatedmortar. The powder was placed in the alumina bowl and the reactor again,and thermal CVD was conducted under the same conditions. The black massthus obtained was similarly disintegrated on the automated mortar again.There was obtained a black powder having a total carbon content of 59%and a free carbon content of 41%. The black powder was then placed in analumina bowl, and heated in air at 800° C. for one hour for oxidizingand removing free carbon on the surface, yielding a blackish gray finepowder.

The fine powder had a total carbon content of 19.3%, a free carboncontent of 1.8%, an oxygen content of 1.8%, and an average particle sizeof about 1.1 μm. On x-ray analysis, a diffraction line attributable tosilicon was found. Upon treatment with a hydrofluoric acid/hydrogenperoxide mixture, a precipitate in the solution was observed, and afterevaporation to dryness, a grayish green residue was observed. These dataconfirmed the structure in which the surface of a silicon particle iscoated with silicon carbide.

From the analytical data, the powder was found to have a composition of39.9% silicon and 58.3% silicon carbide.

As in Example 2, this silicon composite powder was examined by acharge/discharge test as a lithium ion secondary cell negative electrodematerial. The results are also shown in Table 1.

Comparative Example 1

The silicon fine powder resulting from pulverization in Example 2 wasfed to a rotary kiln reactor, and thermal CVD was conducted in a streamof a methane-argon mixture at 1,200° C. for an average residence time ofabout 2 hours. The black mass thus obtained was disintegrated on anautomated mortar. The powder was fed to the rotary kiln reactor again,and thermal CVD was conducted in a stream of a methane-argon mixture at1,200° C. for an average residence time of about 2 hours. The black massthus obtained was similarly disintegrated on the automated mortar again.There was obtained a black powder having a total carbon content of 47.2%and a free carbon content of 36.3%. As in Example 2, this black powderwas examined by a charge/discharge test as a lithium ion secondary cellnegative electrode material. The results are also shown in Table 1.

Comparative Example 2

The silicon fine powder resulting from pulverization in Example 2 wasfed to a vertical tubular furnace (inner diameter ˜50 mm), and thermalCVD was conducted in a stream of an acetylene-argon mixture at 800° C.for 3 hours. The resulting black mass was a carbon-CVD-treated siliconpowder, which on analysis had a total carbon content of 41% and a freecarbon content of 40%. The black mass was disintegrated on an automatedmortar, after which free carbon was oxidized and removed as in Example2.

The fine powder had a total carbon content of 2.3%, a free carboncontent of 1.8%, an oxygen content of 1.3%, and an average particle sizeof 4.1 μm. On x-ray analysis, a diffraction line attributable to siliconwas found. Upon treatment with a hydrofluoric acid/hydrogen peroxidemixture, little precipitate was observed, and after evaporation todryness, no residue was observed.

From the analytical data, the powder was found to have a composition of96.5% silicon and 1.7% silicon carbide.

As in Example 2, this powder was examined by a charge/discharge test asa lithium ion secondary cell negative electrode material. The resultsare also shown in Table 1. TABLE 1 Comparative Example Example Siliconcomposite 2 3 1 2 Average particle size (μm) 4.0 1.1 4.9 4.1 Totalcarbon content (wt %) 12.1 19.3 47.2 2.3 Free carbon content (wt %) 1.31.8 36.3 1.8 Amount of silicon carbide coated 36.0 58.3 36.3 1.7 (wt %)Silicon content (wt %) 62.7 39.9 27.4 96.5 Test results Initial chargingcapacity* (mAh/g) 1570 1210 1050 2630 Initial efficiency* (%) 93 90 8989 Cycle retention after 50 cycles (%) 93 90 89 25*calculated based on the total weight of negative electrode filmincluding graphite

Japanese Patent Application No. 2004-195586 is incorporated herein byreference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. A silicon composite comprising silicon particles whose surface is atleast partially coated with a silicon carbide layer.
 2. The siliconcomposite of claim 1 wherein the silicon particles have an averageparticle size of 50 nm to 50 μm and the silicon particle surface is atleast partially fused to silicon carbide.
 3. The silicon composite ofclaim 1 wherein a diffraction line attributable to silicon is observedwhen analyzed by x-ray diffractometry, and which contains free carbon inan amount of up to 5% by weight.
 4. The silicon composite of claim 1which contains 5 to 90% by weight of zero-valent silicon capable ofgenerating hydrogen gas when reacted with an alkali hydroxide solution.5. The silicon composite of claim 1 wherein after the silicon compositeis treated with a mixture of hydrofluoric acid and an oxidizing agentand heat dried, silicon carbide is left as the evaporation residue.
 6. Amethod for preparing the silicon composite of claim 1, comprising thesteps of subjecting a silicon powder to thermochemical vapor depositiontreatment with an organic hydrocarbon gas and/or vapor at 900° C. to1,400° C., and heating the powder for removing a surface excess freecarbon layer through oxidation.
 7. A negative electrode material for anon-aqueous electrolyte secondary cell, comprising the silicon compositeof claim 1.